1. Field of Invention
The present invention relates to centrifugal pumps and more particularly, to shrouded inducers for centrifugal pumps having means for avoiding cavitation damage from the recirculation of flow about the shroud.
2. Discussion of the Prior Art
It has been found that the addition of a shroud to an otherwise shroudless inducer arrests the formation of vortices at or about the tips of the inducer blades and thusly avoids the cavitation damage to the inducer associated with such vortices. However, the addition of a shroud creates problems of its own in that a portion of the fluid downstream of the inducer tends to recirculate about the outer periphery of the shroud to re-enter the main flow just upstream of the inducer blades. As the recirculating fluid emerges from behind the forward lip of the shroud it often sheds vortices which impinge directly upon the more radially outward portions of the inducer blades. These shroud vortices thusly create an erosive action upon the afflicted portions of the blades and will cause the inducer to suffer similar losses in efficiency and structural integrity as with the aforementioned tip vortices. In this way, the impetus for providing a shroud to avoid the problems associated with tip vortices is compromised by the problems associated with vortices shed at the forward lip of the shroud.
In attempting to meet this problem, the prior art has provided shrouded inducers with labyrinth seals which are implaced about the outer periphery of the inducer shrouds to minimize the flow being recirculated over the shroud. However, no matter how good the labyrinth seal, there is always some amount of flow which passes under the seal to then cause the aforementioned problems. Moreover, as time goes by labyrinth seals tend to lose their sealing effectiveness, especially in pumps where vibration and thermodynamics subject the seal to any degree of rubbing. Of course, an extensive use of labyrinth seals might be employed to reduce the recirculated flow to an absolute minimum, such as is done in the device of U.S. Pat. No. 2,984,189, but such extensive use is impractical and costly. Thusly, there has remained great interest in the discovery of a means of constructing a shrouded inducer which is not subject to the aforementioned problems associated with vorticles emanating from the shroud.